In a small bowl, mix the gelatin with 2 ounces (¼ cup) cool tap water and vanilla extract. If using a vanilla bean, split lengthwise with a paring knife, run the flat of the blade down each half to scrape out the seeds, and add to the gelatin without stirring. Reserve the pod for another project.

AaCombine remaining 6 ounces (¾ cup) water, corn syrup, sugar, and salt in a 3-quart stainless steel pot and set over medium heat. Stir mixture with a fork until bubbling, about 5 minutes, then increase heat to medium-high. Clip on a digital thermometer and cook, without stirring, until the clear syrup registers 250°F, about 8 minutes.

Transfer thermometer to the bowl of a stand mixer and pour in the hot syrup all at once, scraping the pot with a heat-resistant spatula. Cool to exactly 212°F, about 8 minutes, then add gelatin. With the whisk attachment, mix on low speed until the gelatin is melted, then increase to medium-high and whip until thick, snowy white, roughly tripled in volume, and beginning to ball up around the whisk, about 10 minutes.

Scrape into a greased 4-cup container, cover tightly, and let stand at cool room temperature until thick and firm, at least 2 hours, or up to 1 week.

Transfer marshmallow base to the bowl of a stand mixer fitted with the whisk attachment. Whipping on medium speed, begin adding the butter 1 tablespoon at a time, waiting for about 5 seconds after each addition. The fluffy creme will cling to the whisk at first but loosen as the butter is incorporated. Once combined, scrape the bowl with a flexible spatula and whip a minute more. The buttercream should be light and creamy but thick enough to hang upside down from a spoon.

If it seems stiff or dense (feeling greasy rather than melting on your tongue), scoop a cup into a small bowl and microwave until completely melted, about 30 seconds.

Return the melted buttercream to the bowl and whip 15 seconds on medium-high. Conversely, if it seems loose or gooey, refrigerate the entire bowl 15 minutes, then whip 3 minutes on medium-high.